Fuel cell assemblies employing proton exchange membranes are well known. Such assemblies typically comprise a stack of fuel cell modules, each module having an anode and a cathode separated by a catalytic proton exchange membrane (PEM), and the modules in the stack being connected in series electrically to provide a desired voltage output. Gaseous fuel, in the form of hydrogen or hydrogen-containing mixtures such as “reformed” hydrocarbons, flows adjacent to a first side of the membrane, and oxygen, typically in the form of air, flows adjacent to the opposite side of the membrane. Hydrogen is catalytically oxidized at the anode-membrane interface, and the resulting proton, H+, migrates through the membrane to the cathode-membrane interface where it combines with anionic oxygen, O−2, to form water. Protons migrate only in those areas of the fuel cell in which the anode and cathode are directly opposed across the membrane. Electrons flow from the anode through an external circuit to the cathode, doing electrical work in a load in the circuit.
A fuel cell assembly typically comprises a plurality of fuel cell modules connected in series to form a fuel cell stack. For convenience in manufacture, and to provide a more rugged assembly, the anode for one cell and the cathode for an adjacent cell typically are formed as rigid plates and then bonded back-to-back, forming a “bipolar plate”, as is well known in the art. A fuel cell assembly thus consists typically of a stack of alternating bipolar plates and proton exchange membranes. At the outer edges of the assembly, the plates and membranes are sealed together to contain the reactant gases and/or coolant within the assembly. Thus, an important aspect of forming a stacked fuel cell assembly is preventing leakage between the membranes and the plates.
One prior art approach has been to mold a liquid silicone rubber (LSR) gasket directly onto the bipolar plates using liquid injection molding techniques. This has proved to be difficult due to the complex shape of the seal and plate geometry, and also the very brittle nature of some composite materials typically used in forming the bipolar plates.
Another prior art approach has been to provide a die-cut or separately-molded gasket on one side of the plates, the membrane thus being sandwiched between the gasket and the adjacent bipolar plate. In some instances, an assembly may leak initially at the interface between the membrane and the non-gasketed plate surface, although the leak may self-seal when the membrane becomes hydrated in use. Initial leakage, however, is unacceptable.
Thus, sealing means on both sides of each bipolar plate is desirable because a membrane is thus sealed on both its sides against sealing material rather than against a bare bipolar plate.
It is a principal object of the present invention to economically and reliably seal a proton exchange membrane against a bipolar plate surface in a fuel cell stack, both initially and during extended operation of the stack.